Dynamic atomic force microscopy using the frequency modulation technique is investigated for the case that the excitation amplitude is kept constant. This mode of operation has very unique properties. A computer simulation is used to investigate the distance dependence of the measurement signals frequency and amplitude using various conservative and non-conservative interaction forces. It is shown how the two measurement channels are interlinked and influenced by both conservative and dissipative interactions. Further, discontinuous frequency shift versus distance curves as reported in the literature were not obtained. (C) 2002 Elsevier Science B.V. All rights reserved.